The invention relates to a high-pressure sodium discharge lamp provided with an elongate discharge vessel which encloses a discharge path and which vessel has an effective length L.sub.e and, over at least 2/3 of the discharge path, a cross-section S of constant area, in which vessel two electrodes are arranged each having an end between which ends the discharge path extends, this lamp being suitable to be operated in operative condition with a power of periodically alternating value, this power comprising one or more power components sinusoidally varying with time and at least one component having a frequency .nu..sub.i for which it holds that -0.45.ltoreq.2.35.nu..sub.i Le/c.ltoreq.0.45+i where i is an integral positive number and c the average speed in m/s of propagation of sound waves through the gaseous part of the filling of the discharge vessel in the operative condition of the lamp. The invention further relates to an arrangement suitable to operate such a high-pressure sodium discharge lamp.
Discharge lamps are frequently operated with an alternating voltage source, for example, at the usual mains frequency. It is also known to operate lamps at higher frequencies. In case of such an alternating voltage operation, the lamp consumes a power of periodically alternating value. As is known, each power of periodically alternating value can be represented by means of Fourier transformation as a series of power components of mutually different frequencies sinusoidally varying with time, which series may also comprise a power component of constant value.
An elongate discharge vessel is to be understood in this description and in the appended claims to mean a vessel, the efffective length L.sub.e of which and the largest inner diameter D of the part of the discharge vessel having a cross-section S of constant area satisfy the relation Le/D.gtoreq.2. For a circular cross-section S, the largest inner diameter D will correspond to the inner diameter d of the circular cross-section. The effective length Le of the discharge vessel is the quotient of the volume enclosed by the discharge vessel and the surface area of the cross-section S of constant area. The effective length Le is to be considered as being composed of that part of the length between the ends of the electrodes over which the discharge vessel has a cross-seciton S of constant area plus the length of remaining end volumes of the discharge vessel standardized with respect to this cross-section. The expression end volume is to be understood herein to mean the volume enclosed by the discharge vessel near an electrode minus the volume occupied by the electrode.
The average speed c of propagation of sound waves through the gaseous part of the filling of the discharge vessel is determined by the relation (c.sub.p /c.sub.v).sup.1/2. (RT/M).sup.178 , in which: c.sub.p /c.sub.v is the ratio of specific heat at constant pressure and specific heat at constant volume of the gaseous part of the filling of the discharge vessel, R is the universal gas constant (8.313 J mol.sup.-1 K.sup.-1) T is the mean temperature of the gaseous part of the filling of the discharge vessel in K, and M is the mean weight per mole of the gaseous part of the filling of the discharge vessel, expressed in kg/mol.
In high-pressure sodium vapour discharge lamps, the said speed of sound is approximately 500 m/s and the mean temperature T is approximately 2500 K. The mean weight M per mole of the gaseous part of the filling then is of the order of 0.15 kg/mol.
The expression "operative condition of the lamp" is to be understood to mean herein the situation in which the stable discharge is maintained between the electrodes, while the expression "in-operative condition of the lamp" is to be understood to mean the situation in which no discharge takes place between the electrodes.
when a lamp is operated with a power of alternating value, pressure variations will occur correspondingly in the gaseous part of the filling of the discharge vessel. In certain circumstances, this may lead to the occurrence of standing pressure waves. This phenomenon is known as "acoustic resonances". Due to the acoustic resonances, the discharge may be forced out of its position. Arc instabilities are then obtained. When the discharge is forced out of its position, this results in variations of lamp properties and may even result in that the lamp extinguishes.
A lamp of the kind mentioned in the preamble is known from the U.S. Pat. No. 4,052,636. It is suggested in this known patent specification to prevent the occurrence of arc instabilities due to longitudinal acoustic resonances when the known lamp is operated with a power of periodically alternating value by choosing the distane between the electrode ends to be smaller than 0.8 times the length of the discharge vessel. The lamp is then operated at unidirectional voltage pulses at a repetition frequency of 1 kHz and 667 Hz and with a pulse duration of 20%. Experiments have shown that the measure as described in the said U.S. patent specification has only a limited use. With the power forms described in the said patent specification, the means suggested prevent the occurrence of arc instabilities due to longitudinal acoustic resonances, it is true, but it has been found that, when such a high-pressure discharge lamp is operated with other power forms, arc instabilities due to longitudinal acoustic resonances will still occur. The filling in the discharge vessel of the known lamps has a comparatively low pressure. It may in fact be deduced that the pressure of the filling in the discharge vessel of the known lamp in the operative condition is not higher than 155.times.10.sup.3 Pa, the pressure of the sodium being not higher than 20 .times.10.sup.3 Pa.
High-pressure sodium vapour discharge lamps are generally used in public illumination, such as street illumination, because they have a high luminous efficacy. If no particular measures are taken, however, these lamps are not particularly suitable for interior illumination, for example in sports halls, and are certainly not suitable to be used in the domestic field because their color rendition is less satisfactory. A light source suitable for interior illumination namely requires that the general color rendition index Ra of the emitted radiation is at least 60.
It is known that the general color rendition index Ra reaches a value desired for interior illumination purposes if the pressure of the sodium in the operative condition of the lamp is higher than in the case of the known lamp, i.e. at least 30.times.10.sup.3 Pa, the pressure of the filling in the discharge vessel then being correspondingly higher.
It has been found that the occurrence of arc instabilities due to acoustic resonance in the discharge vessel is strongly dependent upon the pressure of the filling, a higher pressure leading more readily to the occurrence of arc instabilities.
The invention has for its object to provide a measure by means of which the occurrence of arc instabilities due to longitudinal acoustic resonances is prevented even at higher pressures of the filling.